1. Field of the Invention
The present invention is directed to a method for testing a conic aspheric microlens, more particularly for measuring a conic constant of the aspheric microlens, and even more particularly for determining an optimal process for making the aspheric microlens.
2. Description of Related Art
Measurement of the surface shape of a conic microlens can be made using an interferometer, such as a Twyman-Green interferometer 10 shown in FIG. 1. The interferometer 10 is capable of measuring the shape of a wavefront very precisely. A light source 12 provides coherent light to the interferometer 10. The interferometer 10 includes a beam splitter 14, a mirror 16, a driver 18, here shown as a piezoelectric element, an objective lens 20, a detector 22, here shown as a charge coupled device (CCD) camera, and a processor 24, here shown as a personal computer (PC).
Light from the light source 12 is divided by the beam splitter 14. Some of the light is directed onto the mirror 16 and reflected back to the beam splitter 18. The rest of the light is directed to an objective lens 20 to be focused onto an object under test, which reflects the light back to the beam splitter 14. The light from the mirror 16 and the object under test interfere at the beam splitter 14, which directs the interference pattern to the detector 22. The detector 22 outputs a signal representative of the interference pattern to the processor 24. The processor 24 then analyzes the data and determines desired parameters of the test object. The mirror 16 is on a translatable stage and its position is controlled by the processor 24 via the driver 18. The object under test may also be translated.
When a microlens 30 is placed in the interferometer 10 as the object under test, the reflected wavefront from the microlens surface can be measured. This measurement is typically performed using a spherical converging wavefront, produced using the objective lens 20. This is useful for measuring spherical lenses because the final measured wavefront is essentially a map of the deviation from sphere of the lens. The goal in this case would be to fabricate the lens to eliminate this deviation. An example of a commercial instrument that performs such a measurement is the MicroLupi metrology system produced by Zygo Corporation. However, the current metrology systems are not capable of measuring the shape of an aspherical lens.
The present invention is therefore directed to a method of measuring aspheric refractive microlenses which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
It is an object of the present invention to provide a way to accurately measure the base sphere (or vertex) radius of curvature of a conic aspheric refractive microlens surface.
It is a further object of the present invention to provide a way to determine the conic constant of such an aspheric microlens surface.
It is yet a further object of the present invention to use the conic constant measurement to optimize the process of making aspheric microlenses.
At least one of the above and other objects may be realized by providing a method of evaluating an aspherical microlens including aligning the aspherical microlens in an interferometer, projecting a spherical wavefront on the aspherical microlens, detecting a wavefront reflected from the aspherical microlens, and deriving from the wavefront a conic constant that specifies the asphericity of the aspherical microlens.
The conic constant from the wavefront may be compared to a desired conic constant. The deriving may include fitting an equation representing an aspheric lens to the wavefront. The equation may be given by       z    ⁢          (      r      )        =                    R        c            ⁢              r        2                    1      +                        1          -                                    (                              1                +                k                            )                        ⁢                          R              c              2                        ⁢                          r              2                                          
where Rc is the base sphere radius of curvature, r is the radial coordinate, z(r) is the surface height at r, and k is the conic constant. The deriving further may include determining the radial coordinate r by calibrating a lateral dimension of the wavefront. The deriving may include expressing the wavefront numerically as a set of polynomials. The set of polynomials constitutes Zernike polynomials. The conic constant derived from Zernike polynomials may be directly proportional to the cube of the base sphere radius of curvature and is inversely proportional to the fourth power of the diameter of the analysis aperture.
The method may further include determining the diameter of the analysis aperture by calibrating a lateral dimension of the wavefront. The method may include establishing a correct confocal position for the aspherical microlens. The method may include calibrating a lateral dimension of the wavefront. The calibrating includes providing controlled radial structures to thereby map positions in the wavefront to lateral coordinates on an object under test. The method may include altering manufacturing of the aspherical microlens in accordance with a difference between the conic constant and the desired conic constant. The method may include reiterating the altering until the conic constant is within a satisfactory range of the desired conic constant.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the present invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility without undue experimentation. Thus, the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.